On Mon, May 20, 2019 at 02:07:09PM +0200, Christian Borntraeger wrote: > > > On 20.05.19 13:51, Paolo Bonzini wrote: > > On 17/05/19 19:48, Marcelo Tosatti wrote: > >> > >> The cpuidle_kvm driver allows the guest vcpus to poll for a specified > >> amount of time before halting. This provides the following benefits > >> to host side polling: > >> > >> 1) The POLL flag is set while polling is performed, which allows > >> a remote vCPU to avoid sending an IPI (and the associated > >> cost of handling the IPI) when performing a wakeup. > >> > >> 2) The HLT VM-exit cost can be avoided. > >> > >> The downside of guest side polling is that polling is performed > >> even with other runnable tasks in the host. > >> > >> Results comparing halt_poll_ns and server/client application > >> where a small packet is ping-ponged: > >> > >> host --> 31.33 > >> halt_poll_ns=300000 / no guest busy spin --> 33.40 (93.8%) > >> halt_poll_ns=0 / guest_halt_poll_ns=300000 --> 32.73 (95.7%) > >> > >> For the SAP HANA benchmarks (where idle_spin is a parameter > >> of the previous version of the patch, results should be the > >> same): > >> > >> hpns == halt_poll_ns > >> > >> idle_spin=0/ idle_spin=800/ idle_spin=0/ > >> hpns=200000 hpns=0 hpns=800000 > >> DeleteC06T03 (100 thread) 1.76 1.71 (-3%) 1.78 (+1%) > >> InsertC16T02 (100 thread) 2.14 2.07 (-3%) 2.18 (+1.8%) > >> DeleteC00T01 (1 thread) 1.34 1.28 (-4.5%) 1.29 (-3.7%) > >> UpdateC00T03 (1 thread) 4.72 4.18 (-12%) 4.53 (-5%) > > > > Hi Marcelo, > > > > some quick observations: > > > > 1) This is actually not KVM-specific, so the name and placement of the > > docs should be adjusted. > > > > 2) Regarding KVM-specific code, however, we could add an MSR so that KVM > > disables halt_poll_ns for this VM when this is active in the guest? > > The whole code looks pretty much architecture independent. I have also seen cases > on s390 where this kind of code would make sense. Can we try to make this > usable for other archs as well? Will move to drivers/cpuidle/
